1. Field of the Invention
This invention relates generally to analyzing gait symmetry, and more specifically, to quantifying gait asymmetry to determine deviation of actual gait from perfectly symmetrical gait.
2. Background Art
Gait asymmetry generally refers to the extent to which the two sides of the body behave differently during a walk cycle. Symmetry is considered to be an important indicator of healthy gait, and it is one of the first casualties of a gait pathology. Gait symmetry can be compromised due to various factors such as limb asymmetry, injury, use of prosthesis, stroke, cerebral palsy, and other mobility-affecting diseases.
Symmetry can be measured through virtually any measurable or computable gait variable. The value of the gait variable is measured or computed for one side of the body and then for the other side of the body, for a corresponding feature. One may use kinematic variables such as step period and step length or individual joint positions or velocities at specific gait events. Comparison between left and right foot trajectories is also a kinematic approach. Kinetic variables, on the other hand, involve acceleration, force, moment, energy, and power. Analyzing entire trajectories or specific discrete values of joint moment and segment power fall under this category. Symmetry can also be studied by comparing the ground reaction force profiles under the individual feet.
Among the existing gait symmetry quantifiers, algebraic indices and statistical parameters represent two major classes. Algebraic indices include the so-called symmetry index and the ratio index, both comparing bilateral variables such as step period or step length. Notwithstanding their successful use in some cases, both of the algebraic quantifiers suffer from major limitations. For example, since the indices report differences against their average values, if a large asymmetry is present, the average value does not correctly reflect the performance of either limb. Also, parameters that have large values but relatively small inter-limb differences will tend to lower the index and reflect symmetry. Additionally, these parameters depend on discrete variables and are thus unable to reflect the asymmetry as it evolves over a complete gait cycle.
Statistical techniques, such as paired t-tests and principal component analysis, and parameters, such as correlation coefficients, coefficients of variation, and variance ratios, have also been used to measure gait asymmetry. While the statistical parameters do not suffer from the limitations of the algebraic indices, their computation is more involved and their interpretation perhaps less transparent.
Despite the broad agreement in the fundamentals of what constitutes symmetry, there is yet to emerge a consensual objective measure of gait symmetry among the researchers. For a comprehensive review of the background and state of the art of gait symmetry research, see Symmetry and Limb Dominance in Able-Bodied Gait: A Review, H. Sadeghi, P. Allard, F. Prince, H. Labelle, Gait & Posture 12:34-45 (2000).
What is needed is a method to quantify gait asymmetry that overcomes the limitations of algebraic indices while also being less computationally involved and easier to interpret than statistical techniques and parameters.